The former Tatler editor's posh frocks website has more than 3 million unique users and is growing fast. It has spawned a men's equivalent, Mr Porter; a discount store, Outnet; a magazine and TV channel. Massenet now employs more than 1,000 people in London and New York.

Net-a-Porter began life in the kitchen of Massenet's Chelsea studio flat while she was pregnant with her first child. Last year, Swiss luxury goods retailer Richemont, which owns Cartier, bought the company in a deal which valued it at £350m, netting its founder £50m.

Massnet reinvested £15m, still owns 18% and remains very much involved as executive chairman.

The daughter of a British model and an American journalist, she grew up in the US but moved to London after meeting her husband, French hedge fund manager Arnaud Massenet. The couple have now separated, but Massenet credits him for guiding her through the world of corporate finance.

Our panel said: "Net-a-Porter is the place to choose products ahead of any glossy magazine. Massenet broke the mould, blending e-commerce and media."